There are sheathed-element glow plugs for diesel engines. Such sheathed-element glow plugs are operated at vehicle electrical system voltages of approximately 11 V for pre-glow and approximately 14 V for a running engine. These sheathed-element glow plugs are controlled by a glow time controller without voltage-reducing measures. This means that the sheathed-element glow plugs are intrinsically safe and they regulate the current during heat-up in such a way that no critical temperatures are exceeded.
European patent document no. 1 034 400 discusses such sheathed-element glow plugs having a reduced control filament portion. This control filament portion is defined such that, when operated using vehicle system voltages, the sheathed-element glow plug cannot be regulated down to operating temperature, but is operated at a temperature which is higher than the admissible temperature. The design of the control filament causes the sheathed-element glow plug to heat up the combustion chamber of an internal combustion engine very rapidly. The operating temperature is regulated by the glow time controller, which sets the voltage via timing (pulse-width modulation).